A nuclear reactor must be provided with a system to control the reactor output. A number of ways of controlling the excess reactivity that is consciously designed into a nuclear power reactor core are known. These include the use of neutron absorbing control rods that can be inserted into or withdrawn from the reactor core, the adjustment of moderator temperature which changes the density and therefore both the fast neutron moderation and the thermal neutron absorption rates of the hydrogen in the light water coolant/moderator, and the use of solid and dissolved neutron absorbing poison materials incorporated either directly in the reactor core lattice (as burnable poison rods or fuel pellet coatings) or dissolved in the primary coolant/moderator as a "chemical shim". The chemical shim is commonly a boric acid solution. Systems employing such a boric acid solution for control of the nuclear reactor are discussed in U.S. Pat. No. 3,380,889 to Loose and U.S. Pat. No. 3,666,626 to Gramer et al. These coolant systems utilize natural boric acid solutions, which contain a maximum boron-10 (B-10) to boron-11 (B-11) atomic ratio of 19.8:80.2.
The prior art has refined processes for concentrating the natural boric acid solutions used as chemical shims in reactor coolant systems. This concentration is necessary due to the need for a highly concentrated solution of neutron capturing compounds at the start of the reactor cycle and to compensate for the loss of B-10 material (nuclei) during the reactor cycle, and to minimize waste water streams containing radioactive wastes. U.S. Pat. No. 4,073,683 to an der School discloses an ion exchange system to reconcentrate a natural boric acid solution while also producing a dilute natural boric acid solution to control the reactivity in the reactor core. U.S. Pat. No. 4,225,390 to Brown et al. discloses a joint ion exchange and evaporative system to control the reactivity of the reactor core wherein the chemical shim is also natural boric acid. U.S. Pat. No. 4,225,390 also discloses how to load follow the reactor using a natural boric acid solution. These processes deal with "concentrating" a solution of natural boric acid, that is, they raise or lower the amount of natural boric acid in a solution, but do not disclose how to operate a nuclear reactor which utilizes "enriched" boric acid as the primary reactor coolant. The term "enriched" refers to a boric acid solution in which the B-10 to B-11 atomic ratio is above the naturally occurring ratio of 19.8:80.2.
It is known that the B-10 isotope is the only isotope in boron-based poisons that contributes materially to the absorption of excess thermal or near thermal neutrons in reactor configurations. This is due to its relatively large neutron capture cross section in the thermal range. Also, it is known that the presence of any of the boron-based poison compounds in a typical power generating nuclear reactor leads to known deleterious effects, such as corrosion and wear, on other material components of the reactor core and of the associated nuclear steam supply system. Therefore, it follows that marked advantages over the prior art reactor coolant systems containing a chemical shim could be obtained if the B-10 to B-11 isotope ratio could be raised, thereby allowing a significant reduction in the total quantity of the boron-based poison material in the primary reactor coolant system at all times during power operations. Such a system would allow for the control of the nuclear reactor and also would be less deleterious on the physical components constituting the nuclear reactor.